In some cases, a raw natural gas obtained by mining a gas field or the like contains helium gas in addition to oil, hydrocarbons of condensates and the like, and impurities such as water and nitrogen. When a commercial-use natural gas product is produced, there is a case in which a treatment is carried out to remove not only the above-described hydrocarbons and impurities but also the helium gas from the raw natural gas.
As means for separating the helium gas from the raw natural gas, according to a rough classification, the methods, a cryogenic distillation process, an adsorption/absorption process, and a membrane separation process have been already known. The cryogenic distillation process is a separation method by distillation using the difference of the boiling point of gas. In the cryogenic distillation process, a raw natural gas containing helium gas is adiabatically expanded so as to be cooled, and mainly, components other than helium are liquefied, thereby separating the helium gas (for example, refer to PTL 1).
The adsorption/absorption process is a method in which a raw natural gas is brought into contact with a predetermined adsorbent or absorbent so as to adsorb or absorb only helium gas to the adsorbent or absorbent, and helium gas is desorbed from the adsorbent or absorbent using the temperature difference or the pressure difference. In addition, in the adsorption/absorption process, components other than helium are adsorbed by the adsorbent or absorbent so as to increase the concentration of helium, thereby separating the helium gas (for example, refer to PTL 2).
The membrane separation process is a separation method in which, for example, a membrane that selectively separates molecules or atoms according to the sizes or properties thereof is used, and a phenomenon in which mainly helium gas penetrates the membrane when a raw natural gas is brought into contact with a single side of the membrane, and a relatively less pressure than that on the single side is formed on the other side is used, that is, the difference in the membrane penetration rate among components in the raw natural gas is used (for example, refer to PTL 3).
Impurities including helium gas are removed from the raw natural gas at a terminal that is connected with a gas field through a pipe line. The amount of the helium gas significantly varies depending on the production area of natural gas, and raw natural gas produced from several production areas in the world contain an extremely larger amount of helium gas than those from other production areas. When the helium gas is separated from the raw natural gas containing a large amount of helium gas as described above, there is a case in which the treatment of the helium gas obtained through separation poses an issue. That is, the laws and regulations of some natural gas-producing countries prohibit the diffusion of the helium gas separated from the raw natural gas in the atmosphere, and in some cases, request the collection of a predetermined portion or more of helium gas.
Helium has been used in the ultralow-temperature cooling of a superconductor, an MRI in a medical field, and the like, and has been positioned as a valuable natural resource, but the consumption amount thereof is not that great, and therefore the easy supply of helium to the market does not guarantee a commercial success.
When a commercial success is not expectable, and the supply to the market is not possible, it becomes necessary to treat the collected helium gas by any means. For example, when excessive helium gas is stored by constructing a storage facility in the terminal, not only the construction cost of the facility but also the maintenance cost of the facility become necessary, and thus considerable efforts and costs are taken to store the helium gas that has no expectation of shipment as a product. Therefore, in production areas of natural gas containing a large amount of helium gas, natural gas has not been actively produced.